This invention relates to digital data processing and, more particularly, to logic for detecting and preventing propagation of erroneous signals on digital data processing apparatus.
The necessity of checking the validity of signals transferred in digital data processing apparatus has long been appreciated. Since their advent, such apparatus have incorporated parity circuitry for verifying at least the form of signals transmitted within them. Though not capable of checking substantive content, this circuitry signals faults when transmitted data has an improper ratio of zero and one bits.
More recently computer systems have incorporated still better mechanisms for insuring the accuracy of transmitted data. For example, U.S. Pat. No. 4,750,177, assigned to the assignee hereof, discloses a computer system in which redundant functional units (e.g., partnered central processing units) synchronously transmit like data signals onto a common bus. By monitoring the bus, the units can determine whether they and their partners are in agreement and, if not, signal an appropriate fault. Similarly assigned U.S. Pat. No. 4,931,922 discloses an input/output controller that synchronizes signal transfers along a peripheral device bus by generating a bus strobe signal after successfully cross-comparing the output of partnered controller boards.
While the techniques described in the aforementioned patents have proven quite successful, they are typically adapted only for use in synchronous environments. There is accordingly a need for data and control signal verification in asynchronous environments.
Therefore, an object of this invention is to provide systems for digital data processing and, more particularly, for improved detection and prevention of propagation of erroneous data and control signals.
Another object is to provide such systems in a form adaptable to use in asynchronous environments.
Still another object is to provide such a mechanism for the verification of control signals used, e.g., to shut-down and restart computer systems.